1. Field of the Invention
This invention relates generally to an antenna system providing redundant steerable beams and, more particularly, to an antenna system for a satellite that employs a small, phased antenna array that provides selectively steerable beams so that the redundant low noise amplifier in each antenna channel front end in the receiver of fixed antenna array can be eliminated and the antenna noise figure can be improved.
2. Discussion of the Related Art
Various communication systems, such as certain cellular telephone systems, cable television systems, intemet systems, military communication systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and is then retransmitted by the satellite to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequencies. For example, the uplink communications signal may be transmitted at 30 GHz and the downlink communications signal may be transmitted at 20 GHz.
The satellite is equipped with an antenna system including a configuration of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. Typically, the antenna system includes one or more arrays of feed horns and one or more antenna reflectors for collecting and directing the signals. Present antenna systems typically optimize the feed structure for the frequency band of interest and sacrifice other frequency bands. The uplink and downlink signals are typically circularly polarized so that the orientation of the reception antenna can be arbitrary relative to the incoming signal. To provide signal discrimination, one of the signals may be left hand circularly polarized (LHCP) and the other signal may be right hand circularly polarized (RHCP), where the signals rotate in opposite directions. Polarizers are employed in the antenna system to convert the circularly polarized signals to linearly polarized signals suitable for propagation through a waveguide with low signal losses, and vice versa.
FIG. 1 is an illustration of a spot beam satellite 10 orbiting the Earth 12. The satellite 10 includes an antenna system 14 that would include an array of antenna feeds, as would be well understood to those skilled in the art. Each feed is associated with a feed channel that may include separate transmitter and receiver architecture to transmit the downlink signal and receive the uplink signal. The satellite 10 may include multiple antenna arrays to increase or improve the coverage area on the Earth 12. Each feed in the antenna system 14 is configured to define a particular coverage cell 16 on the Earth 12. The feeds are directed to define contiguous cells 16, or provide a selected coverage area somewhere on the Earth 12. Each cell 16 would use signals in a particular sub-band within the uplink or downlink frequency band, or adjacent cells 16 would use different sub-bands at different points in time.
Each separate antenna feed channel includes a receiver front end providing signal amplification, typically by a low noise amplifier (LNA), and frequency down-conversion in a manner that is well understood to those skilled in the art. A failure of an LNA in the receiver front end would result in loss of an uplink signal. Because the LNA is a vital component for providing signal gain, and typically has an unacceptable failure rate, each antenna channel front end often employs a redundant LNA. Suitable switches are employed to switch the redundant LNA into the front end in the event that the main LNA fails. However, because the switch occurs in the front end prior to the signal amplification, the switch adversely adds to the noise figure of the receiver, and significantly affects the gain versus temperature noise (G/T) of the antenna system. Such an increase in the satellite G/T is an important design concern.
In accordance with the teachings of the present Invention, a satellite antenna system is disclosed that employs a small phased antenna array acting as a redundant steerable antenna. Because the phased array is steerable, it can be selectively directed to any of the cells on the Earth in the event that the LNA in an antenna channel fails. The phased array is switched into each antenna channel after the channel front end so that the switch used to switch in the phased array is after the amplification in the channel. Therefore, the phased array switch does not significantly affect the noise figure of the channel front end.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.